1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, utilizing an electrophotographic system or an electrostatic recording system.
2. Related Background Art
Image forming apparatuses utilizing an electrophotographic system are well-known. In an image forming process of such an apparatus, after a surface of a photosensitive drum as a latent image bearing member is uniformly charged by a charging device, an electrostatic latent image is formed on the photosensitive drum by a laser or an LED. Then, the electrostatic latent image is developed with toner as developer to form a toner image which is in turn transferred onto a recording material as a recording medium such as paper.
In color image formation, the above-mentioned process is repeated for magenta, cyan, yellow and black colors, and color toner images superimposed on the recording material is fixed to the recording material by heat to form a color image. During this process, after the transferring, residual toner remaining on the photosensitive drum is removed by an exclusive cleaning apparatus.
By the way, recently, a reduction in cost and compactness of the apparatus have been requested. To this end, a so-called cleaner-less image forming apparatus in which such a cleaning apparatus is not provided around the latent image bearing member has been proposed.
In such a cleaner-less image forming apparatus, after the transferring, in order to remove the residual toner remaining on the photosensitive drum (referred to as xe2x80x9ctransfer-residual tonerxe2x80x9d hereinafter), for example, there has been proposed a method in which, after the transferring, a small amount of toner remaining on the photosensitive drum is once received by the charging device as charging means of contact type to change the electrostatic property and then the toner is returned to the photosensitive drum again, and, thereafter, the toner is collected by a developing apparatus also acting as collecting means to use the toner again. By this method, the residual toner on the surface of the photosensitive drum is removed and collected in a sheet-to-sheet interval during the print job or within a predetermined time period after the print job. Such a method is described in the Applicant""s U.S. Pat. No. 6,215,967.
However, in such an image forming apparatus, during continuous printing of images having high density, an amount of transfer-residual toner is increased, with the result that the process for returning the toner to the photosensitive drum may not completely follow the process for receiving the residual toner once by the charging device. In such a case, within the charging device, the toner is mixed with a ferrite carrier as a low resistance carrier forming a dielectric brush. If the amount of toner is greatly increased in the charging device, the surface of the photosensitive drum cannot be uniformly charged with a predetermined potential, with the result that desired image density may not be achieved.
Further, even when images having the same density are printed, in the charging device which has once been used, efficiency of toner discharging process will be worsened, in comparison with a new charging device.
In addition, even when only images having low density are printed, in dependence upon the environment, a small amount of toner may be gradually accumulated in the charging device, thereby worsening the charging ability.
An object of the present invention is to provide an image forming apparatus in which collection and discharging of toner can be effected without worsening a charging ability of a charging device.
Another object of the present invention is to provide an image forming apparatus in which a discharging time from charging means can be altered.
A further object of the present invention is to provide an image forming apparatus comprising an image bearing member, charging means for charging the image bearing member, electrostatic image forming means for selectively removing charges on the image bearing member charged by the charging means to form an electrostatic image with toner as a toner image, transfer means for transferring the toner image on the image bearing member onto a transfer material (the charging means temporarily collecting residual toner after the transferring), return bias applying means for applying a return bias for returning the toner collected in the charging means to the image bearing member in a nonimage forming period, and control means for controlling a return bias applying time period of the return bias applying means.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.